Comparison: Among the NSW deep water cones
the closest species is C. cyanostoma. There are three characters
that distinguish the two species. In sculpture, C. cyanostoma has
most or all of the body whorl with spiral grooves, but in C.
sydneyensis these are restricted to lower half or third. C.
cyanostoma is smaller - up to about 24 mm, compared to 34 mm for C.
sydneyensis. In habitat, C. sydneyensis is recorded only from
55-170 m, but for C. cyanostoma, about two thirds of specimens
are beach collected, the remainder being down to 183 m.

Synonymy: This species has been known as Conus
illawarra (Garrard, 1961), but Moolenbeek (1988) has shown this to
be a synonym of C. sydneyensis.